The objective of this program is to test a unifying hypothesis for the action of the fat-soluble vitamins as regulators of protein synthesis in higher mammals. Studies currently are devoted to elucidating the molecular function of vitamin K. The anticoagulated state, whether induced by coumarin drugs or by vitamin K deficiency is characterized by the appearance of inactive congener proteins closely related to the vitamin K dependent factors which we have called isoprothrombins. We favor the view that a genetic regulatory mechanism exists for switching messenger RNA translation to yield either prothrombin or isoprothrombin. To test these hypotheses, we propose to isolate and translate the messenger RNA for prothrombin and isoprothrombin in a heterologous system and identify the product by specific enzymatic degradation. Furthermore, we propose to isolate and characterize the isoprothrombin from three species (cow, rat, and chick) and to determine the amino acid sequence of their N-terminal fragments liberated by thrombin. The sequence will be compared to the corresponding prothrombin. We propose to attempt to isolate the vitamin K-and coumarin-binding protein which is catalytic in the regulation of prothrombin biosynthesis. Finally, we hope to reconstitute the system for the biosynthesis of prothrombin in a cell-free environment and demonstrate that the regulatory protein is sensitive to levels of vitamin K and warfarin.